(a) Field of the invention
The present invention relates to a tundish of use in a continuous casting plant to continuously cast metal, especially steel.
(b) Brief description of the prior art
It is of common practice in any continuous casting process to use a piece of equipment called "tundish", for separating slags and other contaminants from the molten metal to be cast. Such a tundish is an intermediate vessel which is positioned between a casting ladle containing the molten metal to be cast, and a casting mold which is usually vertical. The molten metal is poured from the ladle into the tundish at one end thereof. The molten metal then flows along the vessel toward the other end thereof. The length of the vessel is selected to provide a time of residence of the metal in the tundish sufficient to allow separation of the inclusions as a floating slag layer. At the other end of the tundish, the molten metal free from its inclusions flows through a vertical outlet tube into the mold from which the solidifying cast slag is continuously drawn out.
Numerous studies have been made up to now to optimize the configuration and design of the existing tundishes, in order to achieve better inclusion separation, less slag entrainmment into the mold, less skulling and higher metallic yield.
The solutions that were proposed up to now to immprove flotation of the inclusions, have been:
(1) to increase the size of the tundish and more particularly its length; and/or
(2) to add partitions, dams and weirs into this tundish (see, for example, U.S. Pat. Nos. 3,814,167 and 4,125,146) to increase the residence time of the molten metal in the tundish.
Each of these solutions have some advantages but none of them is actually adequate to achieve proper inclusion separation.
On the one hand, the use of conveniently located partitions, dams or weirs is certainly useful to increase the residence time of the metal in the tundish. However, such partitions weirs or dams generally create downstream stagnant volumes inside the tundish in addition of being subject to erosion causing contamination of the metal cast. Moreover, in the particular cases of dams, it is known that the efficiency of the dams increases with their height but that higher these dams are, the higher is the decrease in metallic yield.
On the other hand, increase of the width and length of the tundish is also certainly efficient to increase the residence time. However, such an increase of the size of the tundish is not recommended as it leads to greater metallic loss at the end of the casting process and greater dead volumes.
The present invention is based on the recognition that what is actually needed to obtain maximum inclusion flotation is to force the main metal stream in the upward direction toward the slag layer at the surface of the tundish. Such a recognitition has already been made by others (see, for example, U.S. Pat. No. 3,814,167). However, it has never optimized as now proposed in accordance with the present invention.
The invention is also based on the recognition that once the inclusions have been raised and trapped into the slag layer, it is imperative not to draw them down again through the submerged outlet nozzle into the mold. In this connection, it is well known in this art that vortexing above tundish outlet nozzle must be avoided. It is also known that the probability of formation of a vortex increases as the flow of metal, hence the casting speed, increases. To maintain the surface quality of the slab being cast, it is not practical to reduce the casting speed. Therefore, the solutions that have been proposed up to now are:
(1) to keep the metal level in the tundish high, and PA0 (2) to avoid turbulent flows around the nozzle due to convectively induced current.
In the former case, it is of normal practice to keep a head of about 20 cm of metal above the nozzle orifice to avoid slag entrainment. The amount of metal that must remain in the tundish when the same has a flat bottom to maintain this head of about 20 cm, is very costly in terms of metallic yield. Indeed, the larger is the tundish, the higher is the cost, as the amount of metal that may remain "trapped" in the tundish may vary from 3 to 6 tons per sequence in a slab caster of a conventional size (about 4 m long, 1 m wide and 90 cm depth). This, in practice, may represent a loss of about 5,000 tons per year.
In the latter case, it has already been suggested to change the flow characteristic surrounding the nozzle by changing the orifice geometry or by using castellated nozzle or other means. These solutions are however rather costly to reduce into practice.
The invention further is based on the recognition that the use a tundish not as a mere vessel interposed between a ladle and a mold to act as a constant head reservoir, but rather as an open chemical reactor in which reactive alloys such as Ca or CaSi may be added to the molten metal to adjust the metal concentration and minimize the intermixting which usually occurs in the sequential casting, is rather difficult as a plurality of conflicting conditions must be met. Indeed, to proceed to such a "tundish metallurgy", it is compulsory that the tundish be not too deep to float out the newly formed inclusions. However, to avoid vortexing, a sufficient pressure head must be maintained. To homogenize rapidly the alloy additions into the tundish, it is also compulsory that the volume of metal which is subjected to high turbulences and therefore well mixed, must be large enough, which means that the height of injection of the molten metal poured from the ladle must be important. However, the larger the well mixed volume is, the bigger the tundish must be to obtain a reasonable fraction of plug flow volume desirable for inclusion separation. This again mitigates against good metallic yield and increases to possibility of creating dead volumes.